An image displayed on a cathode ray tube (CRT) may suffer from imperfections or distortions such as defocusing or nonlinearity that is incident to the scanning of the beam on the CRT. Such imperfections or distortions occur because the distance from the electron gun of the CRT to the faceplate varies markedly as the beam is deflected, for example, in the horizontal direction. Reducing the defocusing that occurs as the beam is deflected in the horizontal direction, for example, may be obtained by developing a dynamic focus voltage having a parabolic voltage component at the horizontal rate and applying the dynamic focus voltage to a focus electrode of the CRT for dynamically varying the focus voltage. It is known to derive the parabolic voltage component at the horizontal rate from an S-correction voltage developed in an S-shaping capacitor of a horizontal deflection output stage.
A television receiver, computer or monitor may have the capability of selectively displaying picture information in the same CRT using a deflection current at different horizontal scan frequencies. When displaying the picture information of a television signal defined according to a broadcasting standard, it may be more economical to utilize a horizontal deflection current at a rate of approximately 16 KHz, referred to as the 1f.sub.H rate. Whereas, when displaying the picture information of a high definition television signal or a display monitor data signal, the rate of the horizontal deflection current may be equal to or greater than 32 KHz. The higher rate is referred to as 2nF.sub.h. The value n is equal to or greater than 1.
In the horizontal deflection circuit output stage of a video display monitor capable of operating at multi-scan rates, it is known to vary the number of in-circuit S-capacitors using switched S-capacitors. The selection of the S-capacitors is made automatically via selectable switches, in accordance with the selected horizontal deflection frequency.
When a non-switched retrace capacitor is employed, the length of the horizontal retrace interval is the same at different horizontal frequencies. As a result, the required amplitudes of the S-correction voltage at the different frequencies may be different. In a dynamic focus system, it is desirable to maintain the horizontal parabola amplitude constant during the vertical period. It is also desirable to keep the horizontal parabola amplitude constant as the horizontal frequency changes with the scan mode.
In carrying out an inventive feature, a parabolic horizontal rate voltage is developed in the S-shaping capacitor. The parabolic voltage is attenuated through a controlled variable voltage divider. The output of the voltage divider is coupled to an input of a differential amplifier that compares and adjusts the peak-to-peak amplitude of the parabolic voltage to be equal to a voltage reference. The horizontal parabola amplitude is kept constant during the vertical period by comparing the peak to peak amplitude of the parabola to the reference voltage and then the parabola amplitude is adjusted to be equal to the reference via a feedback amplifier and controlled attenuator.
In carrying out an inventive feature, a gain control loop removes unwanted low frequency pin correction modulation from the input voltage from the S capacitor. This modulation, if not removed, can disadvantageously make the horizontal dynamic focus correction too large at the center of the picture. By compensating via the gain control loop, phase error is not introduced.
A further inventive feature is that the high frequency roll off of the focus high voltage amplifier is compensated. The parabola is passed through a low pass filter with roll off similar to the amplifier. The parabola thus attenuated is set equal to the reference. The parabola signal that drives the amplifier is taken ahead of the filter. This signal is boosted at the high frequencies properly to provide a constant amplitude at the amplifier output.
A video imaging apparatus, embodying an inventive feature, includes a cathode-ray tube including a focus electrode. A source of a first parabolic signal at a frequency related to a deflection frequency, selected from a plurality of frequencies, has an amplitude determined in accordance with the selected frequency. A control circuit has an input coupled to the source of the first parabolic signal for generating an output signal. The output signal is for maintaining the amplitude of the first parabolic signal for the plurality of deflection frequencies. An amplifier, that is responsive to the output signal, is coupled to the focus electrode for amplifying the parabolic signal to generate a dynamic focus voltage at the focus electrode.